She doesn't need luck, she needs help. A lot of help.
If you do take her in temporarily, you'll need to set some firm ground rules, like never being allowed to see the former boyfriend who made the threats. I would also tell her to consider some legal means of keeping him away from her and anyone she's associated with because as we have all seen, when someone decides "If I can't have her, NOBODY can!", it gets ugly, fast. If there's some kind of proof of his threats, it needs to be handed over to the police. People who help D-bags like her ex-boyfriend often become victims of their anger/stupidity, so make sure you don't fall into this category.
If her mom is that much or a turd that she'll choose a dirtbag over her own daughter, your "best friend" will have to make some hard decisions, like possibly never seeing her parents again and if it was her dad who raped her, she needs to file a complaint so he can go away.